Background Young people are at high risk of sexually transmissible infections (STI) and notifications of chlamydia are rising rapidly. In 2007, a Victorian multimedia campaign aimed to increase STI testing and condom use among 18–25-year-olds. We conducted a retrospective impact evaluation using multiple sources of routinely collected data. Methods: Population-level chlamydia testing data from general practice, chlamydia testing data from five government primary care clinics with a high caseload of young people, and behavioural data from an annual youth behavioural survey were analysed. Analyses included time-series regression to assess trends in testing levels, Kruskal–Wallis tests to assess changes in positivity, and χ2-tests to assess knowledge and behaviour change. Results: There was no significant difference in the slope of monthly chlamydia testing in population-level or clinic-based surveillance during the campaign compared with before or after the campaign, and no changes in chlamydia positivity. Between 2007 and 2008, there was a significant increase in STI knowledge among females (P < 0.01) and in the proportion of females reporting always using a condom with casual (P = 0.04) and new sexual partners (P < 0.01) in the annual behavioural survey. Conclusions: Our findings suggest the campaign had no impact on STI testing but may have contributed to an increase in knowledge and condom use among females; however, this increase could not be directly attributed to the campaign. Future campaigns targeting young people for STI testing should consider alternative messages and approaches, and include robust evaluation mechanisms to measure campaign impact prospectively.